Consider replacing Courier with Scheduled Message

Courier currently manages scheduled messages, but I have not been able to figure out how to get this to work, or whether it works at all.

Courier is providing the identity plugins for registrants, so cannot be simply removed as a dependency.

I am thinking the identity plugin system should be part of RNG, and eliminate this dependency, possibly moving it to a submodule if anybody wants to continue using Courier for delivery.

Meanwhile, Scheduled Message overlaps Courier by using the queue system to manage messages to be delivered in the future, and providing queue workers to send when the time comes.

I'd like to make these both alternatives workable, if possible, possibly providing submodules to allow a site builder to use the system of their choice (or none at all).